hwmp_recv_perr switches on dest_flags instead of dest_rcode: PERR never actioned

Summary

hwmp_recv_perr(:1768) switch(PERR_DFLAGS(i)) compares 1-bit dest_flags against reason codes 61/62/63. dest_flags can never equal those. Every dest -> default goto done(:1797). Sender correctly uses dest_rcode via PERR_DRCODE(:1901). Received PERRs never invalidate routes. Stale routes persist -> mesh convergence broken.